AGU journal highlights - 10 April 2006

ContentsI. Highlights, including authors and their institutions
II. Ordering information for science writers and general public

I. Highlights, including authors and their institutions

The following highlights summarize research papers in Geophysical Research Letters (GRL) and Paleoceanography (PA). The papers related to these Highlights are printed in the next paper issue of the journal following their electronic publication.

You may read the scientific abstract for any of these papers by
going to http://www.agu.org/pubs/search_options.shtml and
inserting into the search engine the portion of the doi (digital object
identifier) following 10.1029/ (e.g., 2005GL987654). The doi is
found at the end of each Highlight, below. To obtain the full text of
the research paper, see Part II.

1. Terrestrial gamma-ray flashes caused by intracloud lightning

Atmospheric electric field changes, or sferics, are radio waves emitted by lightning discharge processes. Past research has correlated sferics with terrestrial gamma-ray flashes (TGFs), phenomena that occur when short bursts of gamma rays, lasting roughly a millisecond, are emitted into space when electrons traveling at near-light speeds decelerate in the atmosphere. Very little was known, however, about the lightning that produces terrestrial gamma-ray flashes. To shed more light on this, Stanley et al. correlated terrestrial gamma-ray flash data from the Reuven Ramaty High Energy Solar Spectroscopic Imager satellite with sferic data from the Los Alamos Sferic Array and found that all five terrestrial gamma-ray flash sferics detected at sufficiently close range were consistent with intracloud lightning discharge processes that transported electrons upward. In two cases, discharge altitudes were obtained and both were found to be closer to the ground with more attenuating atmosphere between the events and the satellite than previously expected. Furthermore, one of the events was found to lead off an intracloud flash, suggesting that the terrestrial gamma-ray flashes might have been associated with lightning initiation.

When found within the layers of an oceanic core, the abundances of certain marine microfossils can reveal information on past climate in that area. Using records from the Ocean Drilling Program Hole 893A, Pospelova et al. examined the dinoflagellate [single cell organisms with two dissimilar flagella, or tails] cyst microfossil record in Santa Barbara Basin, California, at thousand-year intervals over the past 40 thousand years. They found that changes in cyst abundance, composition, and diversity reflected major shifts in climate and ocean circulation in the region over this time interval. Specifically, significant increases in cyst accumulation rates were found within Holocene [current geological epoch] samples and within samples from times of glacial retreat in the Pleistocene [immediately previous epoch], implying enhanced marine productivity during these periods. By contrast, cyst assemblages from the Last Glacial Maximum [around 21 thousand years ago] exhibit low diversity, with the predominance of the dinoflagellate Selenopemphix nephroides, which is indicative of substantial cooling of surface waters in this area at that time. The authors expect that dinoflagellate cysts will be used to document climate changes in the late Quaternery [current geologic period, including the Holocene and Pleistocene] on the California margin and in other locations.

Title:
Dinoflagellate cysts as indicators of climatic and oceanographic changes during the past 40 kyr in the Santa Barbara Basin, Southern California

Natural volatile organoiodine compounds of marine origin are an important iodine source to the atmosphere. In the air, these compounds are photolyzed [decomposed by light], releasing iodine atoms that take part in tropospheric ozone depletion and contribute to new particle formation in the coastal zone. Martino et al. analyzed the behavior in seawater of one such compound, diiodomethane, to see whether it escapes to the troposphere [lowest part of the atmosphere] in the marine environment. Using natural seawater and a xenon lamp as a radiation source, the authors studied the wavelength dependence of diiodomethane photolysis in the laboratory and then extrapolated to hypothesize on its behavior in nature. They showed that diiodomethane photolyzes in seawater under natural sunlight conditions between wavelengths of 290 and 350 nanometers, with photolysis rates decreasing quickly with depth. They developed a simple photochemical box model of a shallow equatorial mixed layer, which showed that most diiodomethane is photolyzed before reaching the sea surface. Thus, little diiodomethane is released in the remote marine atmosphere, making it less likely to directly affect tropospheric ozone.

Title:
Wavelength-dependence of the photolysis of diiodomethane in seawater

Authors:
Manuela Martino, Peter S. Liss, and John M. C. Plane: Laboratory for Global Marine and Atmospheric Chemistry, School of Environmental Sciences, University of East Anglia, Norwich, United Kingdom.

Similar to terrestrial aurora, high-energy electrons travel along Jupiter's magnetic field into its upper atmosphere, where they ionize gases and emit light. Ultraviolet images of Jupiter's northern aurora, taken by the Hubble Space Telescope clearly show highly volcanic Io as a bright spot interacting with the Jovian atmosphere through its high rate of plasma production. Information about Europa is, however, more difficult to decipher. Grodent et al. studied Hubble's ultraviolet images of Jupiter and confirmed the existence of an electromagnetic interaction between Europa and Jupiter. This auroral signature showed a two-component structure: a roughly circular Europa spot, followed by a previously undetected faint tail emission trailing as Europa orbits Jupiter. This tail extended over 5000 kilometers [3,000 miles], which maps along a region of at least 70 Europa diameters. The authors suggest that auroral interaction at Europa is similar to that at Io, but is scaled down by an order of magnitude, and that the brightness of Europa's footprint depends on its position above or below Jupiter's magnetospheric plasma disk.

Reflection of internal waves from a sloping bottom in the ocean can create locally unstable stratification and wave breaking and thus is thought to play a significant role in ocean mixing. Previous studies have shown that the horizontal component of the Earth's rotation vector, an often neglected constituent of the Coriolis force, can strongly modify the dynamics of low-frequency internal waves, especially in the deep ocean. There, ocean mixing is enhanced above the bottom if the reflected wave nears the angle of the bottom slope, termed the critical reflection angle. Gerkema and Shrira developed equations describing internal-wave reflection that include the Coriolis force in its full form. They found that the occurrence of critical reflection depends on the orientation of the bottom slope, and they applied the new hypothesis to the reflection of internal tides. Examining a range of latitudes and stratification levels, they found that the probability of critical reflection is generally enhanced if the Coriolis force is fully taken into account, especially at higher latitudes.

Title: Non-traditional reflection of internal waves from a sloping bottom, and the likelihood of critical reflection

6. Surface loading due to storm surges can be measured with high accuracy

Surface loading of Earth causes deformations that can be measured using space geodetic techniques and causes gravity variations that can be observed using gravimeters or the space-based Gravity Recovery and Climate Experiment (GRACE). In order to obtain accurate information on crustal movements due to tectonic or post-glacial deformations, the loading deformations due to tides and other atmospheric and oceanic variations must be modeled accurately. As non-tidal ocean deformations are difficult to predict because of the limited accuracy of present-day global ocean models, Fratepietro et al. developed a model of regional ocean loading deformations caused by storm surges. Using the output from a 12-kilometer [7.5-mile]-grid storm surge model on the northwest European shelf, they found that a surge of just over two meters [nearly seven feet] in the southern North Sea produced a vertical displacement in space geodetic measurements of -20 to -30 millimeters [-0.8 inch to -1.0 inch] and increased local gravity by 6–8 microgals over a wide area of northwest Europe. The authors conclude that high resolution non-tidal loading models are needed to correct geodetic measurements made in areas near shallow continental shelves.

Title: Ocean loading deformations caused by storm surges on the northwest European shelf

7. Iron from dust grains is released into the remote ocean by acidic sulfur pollutants

Marine phytoplankton growth, nitrogen fixation, and nutrient cycling are strongly influenced by the dissolved iron content in seawater. Atmospheric transport and deposition of dust particles originating from deserts supply much of this iron and thus are important parameters in carbon uptake through biological production in the ocean. Previous ocean models assumed that the soluble fraction of iron in dust was constant across the globe. Fan et al. questioned this assumption by arguing that sulfur dioxide pollutants attach to individual dust grains, forming a coating; in wet environments the coating hydrolyzes into acids that help break down the grain, dissolving its iron content. This causes the soluble fraction of iron in dust grains to increase with transport time from the source region, moving from about one percent near dust sources to 10-40 percent farther away, with high soluble fractions in remote ocean regions. The authors note that although dust concentrations also decrease with transport time, sulfur dioxide emissions through fossil fuel burning alone could have caused significant iron fertilization in the modern northern hemisphere's oceans.

Modern climate variability in the western Indian Ocean is primarily paced by the El Niño-Southern Oscillation (ENSO) system. Recognizing that climate reconstructions of past environments will improve descriptions of the relationship between Indian and Pacific Ocean climate variability, Damassa et al. analyzed a slow-growing coral, Diploastrea heliopora, off the coast of Tanzania. As the first study to use such species to assess climate variability in the western Indian Ocean, the authors used oxygen isotopes records from the corals, spanning most of the 20th and 17th centuries to generate sea surface temperature records. They found that modern records correlated well with other assessments of climate, documented 20th century warming, and displayed significant power at ENSO periodicities. The 17th century record showed a pronounced interdecadal signal, with sea surface temperatures varying inversely with insolation. The authors note that using this coral will help to further describe the complex relationships between ENSO, its teleconnections, and radiative forcing that existed in the past. When compared with modern climate records, this will be critical in documenting the effects of human influences.
Title:
Enhanced multidecadal variability in the 17th century from coral isotope records in the western Indian Ocean

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